Perhaps a plot where they get magically transported to a parallel universe where they have to pretend to be a couple to blend in. As they play their parts, they start to realize that there are real feelings developing between them. This could involve them having to navigate the cultural differences of this new world while also dealing with their own growing emotions.
One idea could be a story where Natsu and Erza Knightwalker are on a mission together in a strange land. They face numerous challenges that force them to rely on each other more and more, leading to a deepening of their relationship. Their different personalities, Natsu's impulsiveness and Erza Knightwalker's more stoic nature, could create some really engaging character dynamics.
One common theme is the exploration of their relationship in an alternate universe. Since Erza Knightwalker is from a different world in the Fairy Tail lore, fanfictions often play with how Natsu and her interact and fall in love in this new setting.
One idea could be a story where they accidentally swap bodies. This would lead to a lot of comical situations as they try to adapt to each other's lives. They'd have to deal with different relationships within their respective guilds or groups while also learning more about each other's personalities.
Their relationship can be developed through shared adventures. For example, they could team up to save a group of people from a powerful enemy. In the process, they would learn to trust each other more and more, and this trust could be the foundation of a deeper connection.
You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). On these platforms, you can simply search for 'Natsu x Erza Knightwalker' in the search bar, and it should bring up relevant stories. Just make sure to check the ratings and summaries to find the ones that suit your taste.
Perhaps the OC is from a different dimension where magic works in a strange way. Natsu, Mirajane, and Erza are intrigued by this. They team up to help the OC understand and control their magic in the world they are in now. Along the way, they encounter a powerful dark wizard who wants to use the OC's unique magic for evil. So, they have to fight together to protect the OC and the world from this threat.
The potential for a unique relationship. Since they are from different versions of the same world in the Fairy Tail universe, there's a lot of room for creativity in a fanfiction. Writers can explore a romantic relationship, a platonic friendship, or even a rivalry that could turn into something more. The fact that they both have powers and abilities that can complement or clash with each other also adds to the allure of fanfictions about them.
Maybe a fanfiction where Naruto and Erza Knightwalker meet during a tournament. In this tournament, different fighters from various universes are gathered. Naruto uses his Rasengan and shadow clone jutsu, while Erza Knightwalker showcases her different armors and sword skills. They start off as rivals but gradually come to respect each other as they see the similarities in their determination to protect what they love. And in the end, they might even combine their powers to defeat a cheater in the tournament who was using unfair means to win.
One idea could be exploring how their guild mates react to the news. Some might be overjoyed and offer all kinds of support, while others could be a bit shocked at first. Another could be focusing on how Natsu and Erza themselves adjust to the idea of becoming parents. Natsu might be a bit clumsy at first but full of enthusiasm, and Erza could be more practical and start planning everything right away.
One idea could be a story where they go on a quest together. Maybe they have to find a rare magical item. Natsu's impulsiveness, Mirajane's charm, and Erza's strict leadership combine in interesting ways during the adventure.
A theme that often shows up is the discovery of self - worth. For both Natsu and Knightwalker, their relationship can make them see themselves in a new light. For Knightwalker, it could be realizing she can be more than just an enemy. For Natsu, it might be understanding his own capacity for love in a different way. And of course, there's always the theme of magic and how their magical abilities interact in different situations within the fanfiction stories.